In the art of manufacturing pneumatic tires, rubber flow in the tire mold or minor differences in the dimensions of the belts, beads, liners, treads, plies of rubberized cords, etc., sometimes cause non-uniformities in the final tire. Non-uniformities of sufficient amplitude will cause force variations on a surface, such as a road, against which the tires roll producing vibration and noise. When such variations exceed an acceptable maximum level, the ride and handling of a vehicle utilizing such tires will be adversely affected.
Tire uniformity machines are used to monitor the quality of the tire production process and may guide or incorporate corrective measures such as the grinding to improve the balance and uniformity of a tire. In general, a tire uniformity machine subjects a tire to normal conditions of mounting, pressurization, rotation and load while collecting measurement data on variations of deflection, force and velocity. A tire uniformity machine typically includes an assembly for rotating a tire against the surface of a rotating loading wheel. In this testing arrangement, the loading wheel is moved in a manner dependent on the forces exerted by the rotating tire and those forces are measured by appropriately placed measuring devices. When a tire being tested yields less than acceptable results, shoulder and/or center rib grinders are used to remove a small amount of the tire tread at precisely the location of the non-uniformities detected by the measuring devices. In a sophisticated tire uniformity machine, the measurements are stored and interpreted in digital form by a computer and rubber is removed from the tire tread using grinders controlled by the computer. Examples of machines utilizing these methods are disclosed in U.S. Pat. Nos. 3,739,533; 3,946,527; 4,914,869 and 5,263,284.
Unavoidably tire uniformity machines are not themselves perfectly uniform, and so tire uniformity measurement signals may include an erroneous contribution from the tire uniformity machine itself. In effect, minor variations in the design, construction and operation of a tire uniformity machine contribute to variations of deflection, force or velocity that contaminate the tire uniformity measurements with a machine contribution. In general, tire uniformity machines are designed, manufactured and operated to minimize the machine contribution to the tire uniformity measurement data. However there are technical and cost limitations to the precision and care with which a tire uniformity machine can be built and operated such that some machine contribution is inevitable. The prior art provides a variety of examples addressing methods to avoid and correct tire uniformity measurement errors.
As noted by U.S. Pat. No. 4,404,848 ('848), it is often the case that measured values contain errors due to rotational deflections of the rims that grip the inspected tire and/or the load wheel of the tire uniformity machine. In addition small deflections occur due to the deterioration of parts, by rust, or by bruises which are developed during use of the inspecting machines. A method to correct these types of errors is disclosed by the '848 patent wherein the radial runout of the load wheel is measured without a tire in place to obtain an erroneous deflection signal. The erroneous deflection signal is then multiplied by the spring constant of the measured tire and subtracted from the measured value of the radial force obtained from the tested tire.
U.S. Pat. No. 4,404,849 describes a method for correcting errors of measurement due to variations in tire pressure in a tire uniformity inspecting operation.
U.S. Pat. No. 5,614,676 ('676) describes a method of vibration analysis for tire uniformity machines by using signals from load cells when the machine idles. The signals are sent to a computer that outputs an alarm signal when the amplitude of vibration at selected frequencies exceeds acceptable levels.
Since complete elimination of defects and imperfections with the tire uniformity machine is difficult, it is more practical to accept a certain degree of deflections and vibrations while continuing to develop improved methods to monitor and accommodate the inevitable machine contributions.